Certain industries, such as the oil and gas industry, need pumps to pump fluids from a well when the down hole pressure is insufficient to force fluids to the surface. Such industries employ a variety of methods and pumps to pump out wells. For example, some applications employ a rod pump which uses a reciprocating motion to develop a pumping force. Unfortunately, a rod pump develops a pumping force only on the down stroke. In other words, the pump only pumps half the time, that during the down stroke. During the upstroke, the pump barrel of the rod pump is refilled. Also, the volume of liquid that is pumped by a rod is that constrained within the pump barrel and is limited to the displacement of the pump.
Rod pumping, although widely used, suffers from several other drawbacks. For the pump to operate properly, the pump has to be submerged in the liquid being pumped at all times during normal operation. In oil production, whenever the pump is not submerged in the liquid being pumped, the pump sucks in natural gas and it "gas-locks". When the pump "gas-locks," it ceases to do productive work. Because of the close tolerances within the pump and the absence of liquids being pumped, the liquids being pumped lubricate the sliding surfaces within the pump. With the pump empty of any liquids, friction causes the pump to fail. The failure is not immediate but takes place with time. If the "gas lock" condition is discovered early, the pump is stopped, the rods with the internal reciprocating section of the pump are slowly lowered until the reciprocating section within the pump touches an internal check valve. This action compresses natural gas in the pump and can force it out of the pump. The rod is then reset to a new position. The pump is started and checks are made to ensure that the reciprocating internal section of the pump is not striking the internal check valve. This method is called "re-spacing" the pump. This method generally works, but normally the pump does not work at the original programmed rate because the internal wear in the pump increases the tolerances between the reciprocating parts and the consequent increase in slippage of the produced liquids. If "re-spacing" the pump fails, a rod job is required which requires a workover rig.
Another problem with rod pumping is that the pump cannot tolerate produced sand in the liquid that is being pumped. Because of the close tolerances between the plunger and the barrel of the pump, sand causes the plunger of the pump to freeze in the pump barrel. When this happens, the pump ceases to do productive work and a rod job is required to restore production.
In addition to the above problems associated with rod pumps, as the rod and pump accelerates from stop, moving from the upstroke to the down-stroke, and under the force of gravity, the weight and acceleration of the rods and pump during the down-stroke causes an extension which is similar to the extension of a weight on a spring. This extension causes a pounding of the pump on the tubing string. The pounding on the tubing string causes the tubing string to also act as a weight on a spring. If the extensions of the rods and pump on one part and the tubing string on the other part are in phase, the tubing can quickly fail. If the forces are out of phase, the hammering on the tubing string eventually causes the tubing string to leak. If a leak develops due to vibration and/or fatigue, an expensive workover of the production rig is required to restore production.
Another restriction of this type of operation is that rod pumping is limited to straight holes and slightly deviated holes. With the use of rod guides some greater deviation of the hole from vertical can be tolerated. However, a rod pump cannot be used for high angle or horizontal wells.
Also, production is enhanced by increased the pressure differential between production strata and the well bore. However, a rod pump requires submergence and the head of fluid required by the submergence is a positive pressure on the well bore, which significantly limits the rate of producing an oil well.
Another type of pump for pumping fluids from down hole is a rotary rod pump. A rotary rod pump is a progressive cavitation pump which has a rotation motion. The rotary motion is transmitted from a surface motor to the pump via normal sucker rods. This pump is somewhat more efficient than the rod pump and can tolerate some sand and natural gas. But, it is not suitable for highly deviated or horizontal wells or in wells with high gas/liquid ratios or in which formation sand is constantly produced in association with the produced liquid. The rotary rod pump also suffers wear on the rod coupling/tubing area in areas where the rods are in contact with the tubing.
Some applications for withdrawing fluids from down hole call for jet pumping. Jet pumping creates a low pressure area to which the produced liquids migrate, to trap and accelerate the fluid. Jet pumps can handle natural gas without gas lock. Also, jet pumps can reduce the well bore pressure to pressures below normal atmospheric. Unfortunately, known jet pumps cannot handle large quantities or slugs of produced sand. They easily sand up because of the close tolerances through which the production fluids must pass.
Developing well bore pressures below normal atmospheric is important to well performance. The inflow performance of an oil well is dependent upon the pressure differential between the reservoir pressure and the well bore pressure. Thus, the greater the pressure difference, the greater the inflow into the well bore. Vacuum conditions on the well bore provides the highest pressure differential.
In the initial migration of oil from the source rock through cracks and faults to a lower pressure permeable reservoir rock, normally the higher pressure oil would force out water from the reservoir rock and so displace it that the only remaining water would be that water coating the individual sandstone matrix that comprises the reservoir rock. When an oil well is drilled to the reservoir rock, and as oil is produced, the cavity formed when the oil is produced is now taken up with natural gas that comes out of solution from the oil. A drop of oil surrounded by natural gas in a sandstone reservoir, which was initially water wet, has an extremely high surface tension. There is a point in the production of an oil well normally at the time when 15% to 20% of the original oil in place has been produced, especially with reservoirs where the drive mechanism is a secondary gas cap, the oil well cannot be economically produced using known artificial lift techniques. Artificial lift techniques include but are not limited to pumping, i.e., using one of the methods previously described, or gas lift, in which gas is injected into the production string to aerate the column of oil, thereby producing the oil. To produce additional oil at an economic rate, secondary recovery is required. Such currently used secondary recovery techniques include gas injection, surfactant injection, water injection, steam flood, and in situ combustion. At the point in time when it is uneconomic to artificially lift oil wells, the problem is the migration of oil from within the reservoir to the well bore. The oil has a very difficult time getting to the well bore, since it has to pass through the pore spaces in the sandstone matrix and the higher the surface tension between the oil and natural gas interface and the natural gas and the water interface, the lower is the inflow to the wellbore, and the lower is the oil production. It is actually possible for low levels of oil production to occur early in the producing life of the well. It would be advantageous to use an artificial lift technique that develops a vacuum in the well bore to increase the in flow of oil from the formation into the well bore as well as lift the oil at the same time.
Therefore, there remains a need for a down hole pump that can pump liquids, gases, and solids together or individually. Such a pump should not cause or create a rubbing action or a jarring action which may damage the tubing string. Such a pump must also be capable of pumping in highly deviated and/or horizontal wells. A down hole pump ideally pumps 100% of the time, not just on the down stroke as in a rod pump. Such a down hole pump should also be capable of producing vacuum conditions at the well bore or sand face of the reservoir rock. The pumping rate of such a pump should also be capable of being adjusted by using a simple mechanism.
In addition, cost effectiveness of the production operation would be improved if the pump were run down hole initially on the completion string or the completion string were filled with adapters to later accommodate the pump which would then be run into the well using wireline tools and equipment. By running the pump in the well on the initial completion string, the well could be produced earlier than normal because the rig pump can be used with water to circulate out the drilling fluids and induce flow. Also, when the well stops flowing, artificial lift can immediately commence with no interference to the tubing string.